Composite structures sometimes have localized areas containing one or more inconsistencies that may require rework in order to bring the structure within design tolerances.
In the past, one rework process was performed using a patch that was placed over the inconsistent area and secured to the parent structure using mechanical fasteners. This rework technique was desirable because the condition of the patch could be monitored over time by visually inspecting the fasteners. However, the use of fasteners may increase aircraft weight and/or drag on the aircraft, and may be esthetically undesirable in some applications.
In some applications, rework patches have been secured to a parent structure using a bonded joint, however this technique may also require the use of mechanical fasteners that provide secondary load paths forming an arrestment mechanism to limit the growth of an inconsistency. Furthermore, changes in a bonded joint securing a rework patch on a parent structure may not be easily monitored over time because the attaching mechanism of the joint or joint interface may not be visible.
Currently, a single layer bonded repair of airframe components, particularly those fabricated with advanced composite materials, may not be designed nor certified to truly meet the intent of FAR 25-571e and FAR 26 for the future. Since nothing can be seen visually in regards to the quality defect and strength of the resulting bonded joint/repair, the resulting characteristics of the bond and its life may not be known. A process variation may have occurred in the actual installation that may be detrimental to its performance, ability to transfer load efficiently, and one may not be able to detect that variation. Consequently, confidence in the actual performance of a given bond over its life may be lacking. The strength, durability and damage tolerance of a bonded repair may not be determined non-destructively nor have sufficient process controls been developed or put into place for in-service repairs that ensures its integrity by analysis. Mechanically fastened repairs can be visually inspected and confidence in their integrity is assured based on this visual inspection and fail-safe as well as success over time. This may not be the case with regard to bonded joints/repairs.
Existing solutions to primary structure may avoid the reliance of the bonded joint for fail-safe life as there are no current solutions for certifying bonded joints. To meet the FAR requirements including existing and new, secondary load paths may be used and any bonded joint may have a damage arrestment mechanism such as the use of fasteners in the joint/repair. Bonded repairs that restore structural strength to primary structure may not be performed unless specifically authorized and supervised by an engineering authority as well as performed by highly-trained mechanics in a controlled environment. Current repair manuals for in-service repair may limit their application due to this confidence issue, where structural credit may not be given to the patch.
Accordingly, there is a need for a discretely-tailored multi-zone bondline for structural repair which includes a combination of a series of multi-zoned structural bonded joint/repair elements and a uniquely-designed, multi-zoned repair patch/doubler and adhesive combination which results in a manageable and predictable life.